An organic electroluminescence (EL) device is classified into a fluorescent organic EL device and a phosphorescent organic EL device, and a device design optimum for the emission mechanism of each type of organic EL devices has been studied. It is known that, due to emission characteristics, a highly efficient phosphorescent organic EL device cannot be obtained by merely applying a fluorescent device technology. The reason therefor is generally assumed to be as follows.
Specifically, since phosphorescence emission utilizes triplet excitons, a compound used for forming an emitting layer is required to have a large energy gap. This is because the energy gap (hereinafter often referred to as “singlet energy”) of a compound is normally larger than the triplet energy (in the invention, the triplet energy means the difference in energy between the lowest excited triplet state and the ground state) of the compound.
In order to efficiently confine the triplet energy of the phosphorescent dopant material in the emitting layer, it is required to use a host material having a triplet energy larger than the triplet energy of the phosphorescent dopant material in the emitting layer.
In order to reduce the driving voltage of an organic EL device, it is necessary to use a material having excellent charge-injecting properties or charge-transporting properties. However, when a material that is excellent in charge-injecting properties or charge-transporting properties is used, while the driving voltage is lowered, charge balance in an emitting layer may be deteriorated, thus leading to shortened lifetime of a device. That is, a charge-transporting material that is capable of decreasing the driving voltage while keeping the lifetime of a device long is required.
For the reasons mentioned above, in order to improve performance of a phosphorescent organic EL device, material selection and device design different from those of a fluorescent organic EL device are required.
Intensive studies have been made on materials, and several reports were made.
Patent Document 1 discloses a polycyclic compound as a material for an organic EL device. Specifically, it discloses a compound in which polycyclic compounds are bonded through a linking group.
Patent Document 2 discloses, as a material for an organic EL device, a compound in which indolocarbazole skeletons are bonded through an aromatic hydrocarbon group or the like having a structure different from a fused ring structure as a linking group.